ELEMENTAKY PHYSIOLOGICAL CHEMISTRY 179 



(b) Nucleins, etc. Nucleic acid is a compound of alloxuric bodies 

 (hypoxanthin C 5 H 4 N 4 O, xanthin C 5 H 4 N 4 2 , guanin, and adenin) which 

 are basic in nature (see Urine) with phosphoric acid. It does not give 

 the proteid reactions, but seldom exists free, being usually united with 

 albumin to form nuclein. This nuclein, again, is almost invariably 

 combined with another molecule of albumin, the resulting compound 

 being nucleo-albumin. Nuclein is best prepared by digesting nucleo- 

 albumin with gastric juice, whereby the albumin changes into peptone; 

 the peptone goes into solution, and the nuclein, since it is insoluble, is 

 thrown down as a brown precipitate. If the caseinogen of milk be 

 similarly treated, a sediment is also produced, and peptone goes into 

 solution. This sediment is not, however, true nuclein, since on 

 further decomposition, it only yields phosphoric acid and proteid, but 

 no alloxuric bodies. It is hence called Pseudo-nuclein. 



EXPERIMENT XVIII. Take a cellular organ (e.g. thymus gland 

 or pancreas), mince it and then macerate it over-night with water 

 made faintly alkaline by the addition of a drop or two of caustic 

 alkali solution, or of ammonia. Strain the extract through muslin. 

 Add litmus solution to it till it becomes distinctly blue, and 

 then, drop by drop, add weak acetic acid. When the reaction 

 is faintly acid a copious precipitate of nucleo-proteid results, which 

 does not completely dissolve by adding excess of acid (distinguishing 

 it from alkali albumin). Filter off the nucleo-proteid. Add weak 

 alkali to the precipitate, and it re-dissolves. 



Demonstration. The precipitate of nucleo-proteid has been digested 

 with pepsin hydrochloric acid for twenty-four hours; the albumin 

 has been converted into peptone, and the liberated nuclein has fallen 

 down as a brown sediment. 



SCHEMA OF RELATIONSHIPS OF NUCLEIN, ETC. 



Nudeo- Albumin 

 (digested with pepsin) 



Nuclein (precipitated as a brown sediment) Peptone 



(decomposed by acid alcohol) (goes into solution) 



Acid Albumin (in solution) 



Nucleic Acid (a white precipitate) 



(heated in a closed tube with HC1) 



I 



Alloxuric bodies sometimes Carbohydrate Phosphoric Acid 



This nuclein can be further decomposed into nucleic acid and 

 albumin by dissolving it in alkali and then adding O3% hydrochloric 



